We investigated the effects of carotenoids (rigid, rod-like molecules with a length comparable to the membrane thickness but with different polar groups on the ends of the molecule), beta-carotene (non-polar), cryptoxanthin (polar -OH group on one end), zeaxanthin or lutein (polar -OH groups on both ends) on alkyl chain order and motion, phase transition, and polarity of the membrane interior. As model membranes we used saturated phosphatidylcholines (PC) with different alkyl chain lengths (DKPC-12 carbons; DMPC-14; DPPC-16; DSPC-18; and DBPC-22 carbons) and unsaturated PCS, DOPC and soybean PC. We used stearic acid spin labels with nitroxide moiety located at different positions along the alkyl chain. Our results show that anchoring of the carotenoid molecule on one side or both sides of the membrane is significant for effective membrane-carotenoid interaction. Nonpolar beta-carotenoid exerts only minor effects on membrane structure and motion. Preliminary data show, however, that in the presence of membrane-soluble peptides (simple model of membrane proteins) gramicidin effects of beta-carotene are comparable with those of polar carotenoids. [unreadable]